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Sea nettles (Chrysaora fuscescens) float in the water of California's Monterey Bay. The nettle's sting is deadly to small prey but not potent enough to kill humans — except in the case of an allergic reaction.

They sting, even kill, swimmers. They block the cooling systems of power plants. They clog fishing nets and kill penned salmon.

In recent years, reports of havoc caused by swarms of jellyfish have inspired speculation that these simple, otherworldly creatures are capitalizing on changes we have brought to ocean ecosystems.

Scientists are finding that we could be jellyfish's potential benefactors. Overfishing relieves them of competition and predators. Nutrient-rich pollution can cause phytoplankton blooms, providing feasts for some jellies and reducing the water's oxygen content, which could favor their high tolerance for low oxygen. The warmth of climate change could foster expansion among some species. We transport invasive species to new environments, where they thrive. And coastal development provides new shelter for the jellies' stationary life stage, called a polyp.

An article called "The Jellyfish Joyride," published in 2009 in the journal Trends in Ecology and Evolution, discusses the theory that, without a change on our part, these pressures could push ecosystems topped by fish and marine mammals to devolve into ones dominated by jellyfish, as they may have been 500 million years ago.

"So you end up with this kind of imaginary animal that has all these properties, which actually no one jellyfish has," Haddock said. "Basically, jellyfish need to eat, and jellyfish are eaten by things as well, so they are part of a healthy ecosystem."

The term "jellyfish" is a slippery one. In general use, it encompasses two groups: Cnidaria, a diverse group of animals armed with stinging cells, which include corals and true jellies — typically, the gelatinous creatures beachgoers encounter. The others are the Ctenophores or comb jellies, which use rows of tiny hairs, called cilia, to swim — and they don't sting. Fossil evidence of both dates back to the Cambrian Period, which lasted from 543 million to 490 million years ago.

Blooms occur when polyps, the early life stage, of some Cnidaria jellies bud off to form the free-floating, umbrella-shaped medusae, which we think of as jellyfish. The polyps bud simultaneously, and one can produce many medusae, creating — depending on one's perspective — blooms or swarms of jellies. Comb jellies, hermaphrodites that release eggs and sperm simultaneously, can also create blooms when they reproduce.

In Japan, conflicts with jellyfish have climbed in recent years, as populations of moon jellies and the giant Nomura's jellyfish, which grow up to 6.7 feet (2 meters) in diameter, have clogged fishing nets and power-plant intakes.

While there is no doubt that these jellies are showing up more frequently in Japanese waters, it's not clear whether the blooms are more intense than years prior, because scientists cannot determine the population sizes, according to Shin-ichi Uye, a professor at the Graduate School of Biosphere Science at Hiroshima University.

Based on the seeming increase in blooms around Japan and elsewhere in the East Asian seas, Uye suspects the increases are global in scale and attributable to human activities like overfishing, coastal construction and nutrient pollution, as well as warming waters.

"However, I admit that the data are not sufficient enough" to conclude that the phenomenon is global, he told LiveScience in an e-mail. "In fact, I paid no attention to jellyfish until they became increasingly problematic in fisheries in [the] 1990s."

No solid baselineScientific studies of jellyfish increased toward the end of the last century, but they were significantly outpaced by jellyfish stories in the popular press, indicating the hype may be media-driven, said Rob Condon, one of the lead investigators for the National Center for Ecological Analysis and Synthesis jellyfish working group. The group is examining the hypothesis that increases in jellyfish blooms are occurring worldwide.

A conclusion would be premature, according to Condon.

"The big thing here is an 'if,' and the jury is still out on this," he said. "I say, 'Show me the numbers.' Undoubtedly there are localized areas where blooms have increased. ... On a global scale, we don't know enough about jellyfish populations, their biology, their distribution, to make a judgment."

His group is trying to get to the bottom of this, looking at the limited data available both from government and scientific reports, and building a database that will outlive the two-year project. Public outreach is also part of the mission; anyone can submit a sighting on a website (jellywatch.org), and the group is holding an outreach event in Santa Barbara, Calif., on Nov. 20.

Because long-term data on jellyfish populations are limited, Lucas Brotz, a graduate student at the University of British Columbia, also has been looking at anecdotal information — reports in newspapers or from those who work at sea. He said he is seeing evidence of population increases but is not yet prepared to say if they are significant.

"One of the problems with identifying changes in jellyfish populations is they fluctuate with so much variability. One year you see a million, and the next year there won't be any," Brotz said.

No simple answersAlthough researchers have attempted to correlate environmental changes with jellyfish populations, it's often difficult to draw a straight line between them.

There is evidence that some species of jellyfish increase or expand as waters warm. However , a complex scenario played out 10 years ago in the Bering Sea, near Alaska, contradicts that.

Trawl data collected by the Alaska Fisheries Science Center revealed that numbers of jellyfish, primarily the sea nettle, grew rapidly throughout the 1990s in the Bering Sea, while temperatures were moderate. The catches peaked in the summer of 2000, at about 40 times larger than in 1982, and the jellyfish also expanded their range. Then they began to decline, while the sea warmed markedly.

The decline may have occurred when the jellyfish food supply — fish and tiny floating animals called zooplankton — could no longer support their growing appetites, according to a 2008 study in the journal Progress in Oceanography. When conditions for zooplankton blooms were optimal, the jellyfish populations grew.

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A similarly complex relationship between jellyfish and certain fish — which eat each other's young — may give the jellyfish an advantage in overfished waters. This phenomenon may have suppressed the recovery of Namibia's sardine and anchovy fisheries after they collapsed, according to Mark Gibbons, a plankton biologist at the University of the Western Cape in South Africa.

There is also a large unknown factoring into the population changes seen in the Bering Sea, since scientists have little information about where jellyfish polyps bud off to form the familiar medusa and about the conditions needed for the life-stage change.

"Our understanding of the basic polyp life history is almost negligible," Gibbons said. It is known that polyps can stay attached for very long periods of time, and if the environment becomes hostile they can effectively shut down and then "come back to life." They can also bud off more polyps under certain conditions. However, the factors influencing their behavior are poorly understood in most species, he wrote in an e-mail to LiveScience.

Unfairly villainized?Jellyfish blooms are nothing new; these sudden proliferations of medusa are recorded in the fossil record more than 500 million years ago. "So it is hard to know if that is any different than it was a long time ago," according to Haddock.

Haddock, also a member of the NCEAS working group, said he came across a 1925 study of jellyfish reproduction, which the author speculated would help explain the masses of jellyfish that had washed up onto the beach in Monterey Bay. "Even for him in 1925, it went without saying, yeah, we get these big jellyfish blooms all the time."

Census of the seas

The Census of Marine Life has issued its final report on the 10-year effort to document the diversity of the world's oceans. More than 2,700 scientists cataloged 28 million observations of new species as well as old favorites such as the octopus. This octopus specimen was collected at Lizard Island on Australia's Great Barrier Reef in an Autonomous Reef Monitoring System, or ARMS, at a depth of 30 to 36 feet (10 to 12 meters). The ARMS system is one of the legacies of the census.
(Julian Finn / Museum Victoria)
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Jam-packed jellyfish

The ultimate role of a jellyfish is to reproduce. The brown granular core in this jelly, Bouganvillia supercilliaris, is stuffed to the breaking point with hundreds of eggs.
(Russ Hopcroft / Univ. of Alaska-Fairbanks / COML)
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Tree of the sea

Seal of approval

A young Weddell seal (Leptonychotes weddellii) gets his final diving lessons. He is already the size of his mother, who is waiting under the water. Soon, he'll be on his own. The Census of Marine Life cataloged marine mammals as well as other types of creatures ranging from fish to microbes.
(Galatee Films)
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Floating ... and stinging

Sea nettles (Chrysaora fuscescens) float in the water of California's Monterey Bay. The nettle's sting is deadly to small prey but not potent enough to kill humans - except in the case of an allergic reaction.
(Richard Hermann / Galatee Films)
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Vampire of the deep

The vampire squid, also known by the scientific name Vampyroteuthis, is a cephalopod that lives in the oxygen minimum zone of California's Monterey Bay at depths of 2,000 to 3,000 feet (600-900 meters).
(Kim Reisenbichler / MBARI)
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Coral complexity

Researchers with the Census of Marine Life conducted an inventory of octocorals, named for the eight tentacles that fringe each polyp. This is a soft coral, Dendronephthya, from the coral gardens off Lizard Island on Australia's Great Barrier Reef.
(Gary Cranitch / Queensland Museum)
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A string of stomachs

This physonect siphonophore (Marrus orthocanna) was photographed during NOAA’s "Hidden Ocean Expedition." The colonial animal is made up of many repeated units, which include tentacles, and multiple stomachs. Many specimens were observed at depths between 1,000 and 5,000 feet (300 to 1,500 meters).
(Kevin Raskoff / COML)
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Long-haired crab

South of Easter Island, the Census of Marine Life's vent explorers discovered a crab so unusual it warranted a whole new family designation, Kiwidae. Beyond adding a new family to the wealth of known biodiversity, its discovery added a new genus, Kiwa, named for the mythological Polynesian goddess of shellfish. Its furry or hairy appearance justified its species name, hirsuta.
(A Fifis / Ifremer via AP)
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Painful beauty

This type of anemone lives in a mucous tube on the muddy bottoms of coastal waters, estuaries and soft seabeds. They're found in tropical and subtropical waters throughout the world, where they can grow up to 6 inches (15 centimeters) across and a foot (30 centimeters) tall. When the anemone is threatened, the animal retracts into its tube for protection. The beautiful stinging tentacles of the tube anemone vary from a vibrant purple to a creamy brown.
(Karen Gowlett-Holmes / COML)
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Two little urchins

Bottom-dwelling animals often release their larvae into the water for feeding and dispersal as "meroplankton." These two larvae from Russia's Chukchi Sea measure less than a tenth of an inch (1 millimeter) but will eventually grow into sea urchins or sand dollars.
(Russ Hopcroft / Univ. of Alaska-Fairbanks / COML)
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The single snail

This snail (Alviniconcha sp.) inhabits deep-sea hydrothermal vents along the Suiyo Seamount, off the coast of Japan. It's probably a new species, and only a single specimen has been discovered to date. Where are its peers?
(Yoshihiro Fujiwara / JAMSTEC)
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Snazzy snail

The flamingo tongue snail, Cyphoma gibbosum, was photographed near Grand Cayman in the British West Indies, and is listed in the Gulf of Mexico biodiversity inventory.
(Kacy Moody / COML)
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Behold the squidworm

In October 2007, U.S. and Filipino scientists traveled to the Celebes Sea in Southeast Asia, searching for new species living in its deep water. When they discovered this extraordinary worm — which they named "Squidworm" — they knew they had something special.
(Laurence Madin / WHOI)
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Fragile star

The acantharians are one of the four types of large amoebae that occur in marine open waters. Their fragile skeletons are made of a single crystal of strontium sulfate that quickly dissolves in the ocean water after the cell dies.
(Linda Amaral Zettler / MBL (micro*scope))
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Green banners

Nereocystis, a marine alga commonly referred to as bull kelp, is often found in the nearshore and shallow gulf areas of North America's Pacific Coast.
(Brenda Konar / COML)
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Flea gets its close-up

Hyperoche capucinus is a common sand flea that swims in polar waters. This specimen is about the width of a finger.
(Russ Hopcroft / Univ. of Alaska-Fairbanks / COML)
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A different kind of flytrap

This striking creature, a Venus flytrap anemone (Actinoscyphia sp.), was photographed in the Gulf of Mexico. The anemone is so named because it resembles the land-based Venus flytrap, a carnivorous plant.
(Ian Macdonald / Florida State University)
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Alien in earthly waters

Gary Cranitch's photographs for CReefs were recognized for excellence by the Australian Institute of Professional Photographers. This spectacular jellyfish inhabits the water of Australia's Great Barrier Reef off Lizard Island.
(Gary Cranitch / Queensland Museum)
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